The present invention relates to a pendulum acceleration sensor for detecting an acceleration by measuring the swing amount of a pendulum and, more particularly, to an axial support structure for the pendulum.
A pendulum sensor, a leaf spring type sensor, and the like are available as a conventional vehicle acceleration sensor used for an anti-lock brake and a tilt angle sensor for detecting a tilt angle with respect to the direction of gravity. All these sensors are designed as follows. A displacement such as a swing of a pendulum or a bend of a leaf spring due to an acceleration or gravity is detected by an optical sensor, a magnetic sensor, or a capacitance sensor. The detected value is converted into an electrical signal. An acceleration or a tilt angle in an object to be measured, e.g., a vehicle, is then detected on the basis of this electrical signal. A damper mechanism is provided for such an acceleration or tilt angle sensor to prevent a pendulum, a leaf spring, or the like from resonating with external vibrations or to provide proper response characteristics.
As a damper mechanism, for example, a mechanism using a liquid such as a silicone oil or an anti-freeze is available. If a damper mechanism uses such a liquid, the cost of its hermetic structure is very high. In addition, as the viscosity of the liquid changes with a change in temperature, the damping characteristics change. That is, in a damper mechanism using a liquid, it is inevitable that the response characteristics change with a change in temperature. In order to eliminate such drawbacks of a damper mechanism using a liquid, a damper mechanism using magnetism has recently been employed. This magnetic damper mechanism is designed such that permanent magnets are arranged to oppose each other through a plate-like pendulum consisting of a nonmagnetic conductive material. In this structure, the pendulum is braked by an eddy current generated therein as it swings. Such a magnetic damper mechanism is not influenced by a change in temperature and hence has excellent temperature characteristics. A pendulum acceleration sensor having this conventional magnetic damper mechanism will be described below with reference to FIGS. 6 to 8C.
FIGS. 6 to 8A show the schematic arrangement of the conventional pendulum acceleration sensor. Referring to FIGS. 6 to 8A, the pendulum acceleration sensor denoted by reference numeral 10 as a whole is designed such that a pendulum 11 as a swing member swings about a support shaft 12. The pendulum 11 is made of a nonmagnetic conductive material to have a sectorial plate-like shape. A plurality of slits 11a and holes 11b are formed in the rim portion of the pendulum 11. Note that the support base, of the pendulum acceleration sensor 10, which swingably supports the pendulum 11, a detection means for detecting the displacement amount of the pendulum 11 and converting the amount into an electrical signal, and the like are omitted from FIGS. 6 and 7.
Reference numeral 13 denotes a magnetic damper, which comprises a pair of yokes 14, each having a U-shaped cross-section, and a pair of permanent magnets 15 respectively fixed to the yokes 14. The pair of yokes 14 and the pair of permanent magnets 15 are arranged to oppose each other through the pendulum 11. Each yoke 14 is fixed to a support base 16 such that magnetic pole portions 14a on two ends of the yoke face the pendulum 11. The permanent magnet 15 is mounted on the widthwise middle portion of the yoke 14. That is, a magnetic pole surface 14b on an end portion of each magnetic pole portion 14a opposes a side surface of the pendulum 11. Note that each permanent magnet 15 is magnetized in a direction parallel to the axial direction of the pendulum 11. Each yoke 14 is positioned such that the permanent magnet 15 is located immediately below the support shaft 12 of the pendulum 11 and opposes the rim portion of the pendulum 11.
Reference numeral 17 denotes a printed board, which has a rectangular window 17a formed therein and is positioned/mounted on the support base 16. Two pairs of holders 18 for storing two pairs of light-emitting and light-receiving elements as a detection means for detecting the displacement amount of the pendulum 11 and converting the amount into an electrical signal are mounted on the printed board 17 to sandwich the pendulum 11. A pair of opposing shaft support portions 16a defining a swing space for the pendulum 11 extend vertically upward from the support base 16 to extend through the window 17a. The support shaft 12 of the pendulum 11 is supported on the shaft support portions 16a such that the pendulum 11 is swingable in the swing space.
The operation of the conventional pendulum acceleration sensor having the above arrangement will be described next. When an acceleration is applied to the pendulum 11 or the support base 16 is tilted, the pendulum 11 swings about the support shaft 12. As the pendulum 11 swings, the optical paths extending from the light-emitting elements to the light-receiving elements through the slits 11a and the holes 11b are partly blocked. The displacement amount of the pendulum 11 is then obtained as changes in the amount of light received by the light-receiving elements, and is converted into an electrical signal. With this operation, an acceleration is detected. At this time, the pendulum 11 crosses magnetic fluxes generated between the opposing permanent magnets 15 and yokes 14. These magnetic fluxes are indicated by arrows .PHI. in FIG. 7. When the pendulum 11 crosses the magnetic fluxes in his manner, an eddy current is generated in the pendulum 11 consisting of a nonmagnetic conductive material, and the eddy current serves as an eddy current brake for braking the pendulum 11. As a result, the pendulum 11 is immediately stopped at the initial position. With this operation, an acceleration applied next can be detected without requiring a return time.
In the above-described conventional acceleration sensor, the shaft support portions 16a for supporting the pendulum 11 and the holders 18 for storing the light-emitting and light-receiving elements serving as the displacement amount detection means are separately formed. In this structure, owing to the accumulation of manufacturing and assembly errors, the positioning precision of the light-emitting and light-receiving elements with respect to the pendulum 11 deteriorates, resulting in a deterioration in detection precision. In addition, the yokes 14 arranged to sandwich the pendulum 11 are located outside the shaft support portions 16a and hence are respectively spaced apart from the plate surfaces of the pendulum 11 by distances corresponding to the thicknesses of the shaft support portions 16a. That is, in the conventional acceleration sensor, the pair of shaft support portions 16a are formed separately from the holders 18 and are integrally formed on the support base 16 to extend vertically upward and oppose each other. In this structure, since the thickness of each shaft support portion 16a can only be decreased to a certain limit in consideration of the strength, and the distance between each permanent magnet 15 and a corresponding plate surface of the pendulum 11 increases accordingly. For this reason, in order to obtain a predetermined magnetic flux density .phi. on each plate surface of the pendulum 11, one of large permanent magnets 15 are required. Therefore, the device itself increases in size, and the number of components increases, resulting in an increase in cost.